1. Field of the Disclosure
The disclosed disclosure is related to downhole well investigation methods and, in particular, to studying a resistivity distribution of a formation surrounding a borehole.
2. Description of the Related Art
Electromagnetic induction resistivity well logging instruments are well known in the art. Electromagnetic induction resistivity well logging instruments are used to determine the electrical conductivity, and its converse, resistivity, of earth formations penetrated by a borehole. Formation conductivity has been determined based on results of measuring the magnetic field of eddy currents that the instrument induces in the formation adjoining the borehole. The electrical conductivity is used for, among other things, inferring the fluid content of the earth formations. Typically, lower conductivity (higher resistivity) is associated with hydrocarbon-bearing earth formations. The physical principles of electromagnetic induction well logging are well described, for example, in, J. H. Moran and K. S. Kunz, Basic Theory of Induction Logging and Application to Study of Two-Coil Sondes, Geophysics, vol. 27, No. 6, part 1, pp. 829-858, Society of Exploration Geophysicists, December 1962. Many improvements and modifications to electromagnetic induction resistivity instruments described in the Moran and Kunz reference, supra, have been devised, some of which are described, for example, in U.S. Pat. No. 4,837,517 issued to Barber, in U.S. Pat. No. 5,157,605 issued to Chandler et al and in U.S. Pat. No. 5,600,246 issued to Fanini et al.
The conventional geophysical induction resistivity well logging tool is a probe suitable for lowering into the borehole and comprises a sensor section containing a transmitter and receiver and other, primarily electrical, equipment for measuring data to infer the physical parameters that characterize the formation. The sensor section, or mandrel, comprises induction transmitters and receivers positioned along the instrument axis, arranged in the order according to particular instrument or tool specifications. The electrical equipment generates an electrical voltage to be further applied to a transmitter induction coil, conditions signals coming from receiver induction coils, processes the acquired information, and stores or by means of telemetry sends the data to the earth surface through a wire line cable used to lower the tool into the borehole.
Conventional induction well logging techniques employ coils wound on an insulating mandrel. One or more transmitter coils are energized by an alternating current. The oscillating magnetic field produced by this arrangement induces currents in the formations which are nearly proportional to the conductivity of the formations. These currents, in turn, contribute to a voltage induced in one or more receiver coils. By selecting only the voltage component which is in phase with the transmitter current, a signal can be obtained that is approximately proportional to the formation conductivity. In a conventional induction logging apparatus, the basic transmitter coil and receiver coil have axes which are aligned with the longitudinal axis of the well logging device. (For simplicity of explanation, it will be assumed that the borehole axis is aligned with the axis of the logging device, and that this axis defines a vertical direction. Thus, transmitter and receiver coils aligned with the longitudinal axis are “vertically-oriented.”) This arrangement tends to induce secondary current loops in the formations that are concentric with the vertically-oriented transmitting and receiving coils. The resultant conductivity measurements are indicative of the conductivity (or resistivity) of the surrounding formations. Some formations may display anisotropic conductivity profiles, such that the conductivity measured in a vertical direction is different than the conductivity measured in a horizontal direction. This anisotropic conductivity can be detected by using additional coils oriented along axes different from the vertical axis.
A multi-component induction logging apparatus allows for obtaining data related to both vertical and horizontal resistivities and are known in the industry. Typically, such an apparatus contains a set of induction coils oriented in certain directions distributed along the sensor (the mandrel) in a special arrangements or arrays. A typical arrangement has three transmitter coils, with one vertically-oriented and two transversely-oriented (oriented in the plane perpendicular to the longitudinal axis). Typically, these coils define an orthogonal system and may produce magnetic fields substantially oriented along x-, y-, and z-axes, where the z-axis often refers to the vertical axis. The vertically-oriented array radiates a field primarily along the longitudinal direction and measures the formation response in the direction coaxial with the longitudinal axis of the tool. Generally, in a vertical borehole, this array obtains measurements regarding the horizontal resistivity of the formation. Alternately, a radially-oriented (transverse) array radiates a field oriented primarily in the radial direction and generally obtains measurements regarding the vertical resistivity of the formation.
It is known that due to specifics of the well logging instruments design, the mandrel often serves as a load bearing element. The mandrel maintains the tool integrity, carries the load introduced by tools attached below the induction instrument, withstands a significant torque, etc. All the above-mentioned requirements necessitate using a significant number of metal components in the mandrel. These metal components provide a conductive path though the sensor for electrical instruments separated at locations above and below the tool. The presence of metal bodies in the sensor section leads to unwanted axial currents in these metal parts and to the appearance of systematic errors in the instrument response. These systematic errors are often called an “offset.” Methods for addressing these offsets are discussed, for example, in U.S. Pat. No. 6,586,939 to Fanini et al., having the same assignee as the present disclosure. Although the offset problem is severe for radial arrays, it is almost insignificant for vertical arrays.
Another measurement issue encountered in induction logging is called a “borehole effect” and affects the performance of induction tools through an induced current flow that is proximate to the mandrel surface. These currents are magnetically induced or created by a potential difference between upper and lower tool electronic parts due to these parts being exposed to conductive mud. The magnetic fields generated by the induced current often mask useful responses from the formation. The borehole effect can be suppressed by reducing these induced currents. Also, special software post-processing, such as multi-frequency focusing (MFF) can be used to account for the borehole effect. U.S. Pat. Nos. 6,573,722 and 6,624,634, to Rosthal et al., discuss methods for reducing the borehole effect and include, among others, providing a counter-current to the induced current, providing an alternate path for the induced current, and using a superposition technique.
Induction tools, including HDIL (High Definition Induction Logging) which employs multiple vertically-oriented receivers and array-induction logging (AIL) having transmitters and receiver oriented in multiple directions, have been known to encounter the borehole effect in the presence of conductive mud. If the induced current flows entirely in the conductive mud, the effects tend to cancel out. However, where a significant portion of the induced current passes through the formation and the mud is much more conductive than the formation, then this borehole effect can become significant. Generally, the borehole effect occurs in a mandrel decentralized in the borehole such that a standoff presents itself between the mandrel and formation.
Multi-frequency focusing (MFF) is an efficient way of increasing depth of investigation for electromagnetic logging tools. MFF techniques suppress significant portion of the measured signal. Therefore, reducing the borehole effect is important in MFF testing as well as traditional axial tool testing.
The induced current can exhibit a non-uniform current density distribution on the metal surface of the mandrel. Thus, to obtain a corrected measurement typically requires a significant amount of auxiliary data (tool position, borehole shape, invasion profile, etc.). Determining the correct measurement is desired for subsequent calculations, such as heavy 3D modeling of expected tool response and inversion.
The problem of borehole effect can be minimized if this induced current distribution is known or measured. Thus, there is a need to account for the effects of induced currents from a metal mandrel measurements obtained in a borehole with conductive mud. The present disclosure addresses this need.